The present invention relates to an alarm system in which detected information from a detector to detect leakage of a gas, fire, theft, etc. is transmitted to a receiver due to a radio system.
For example, in alarm systems such as a leaked gas alarm, fire alarm apparatus, theft alarm apparatus, etc. which are used at home in group housing or the like, sensors and alarm units are installed indoors and when a sensor detects the abnormal condition, the corresponding alarm unit operates, thereby allowing an alarm buzzer or an alarm bell to generate an alarm sound. At the same time, the alarm unit serves to automatically report to a gas company, fire department, police station, etc. using a telephone line.
On the other hand, when the sensor transmits erroneous detection information due to malfunction, the alarm unit generates an erroneous alarm. To prevent this, the alarm unit alarms after the elapse of a constant time after completion of the transmission of the detection information from the sensor.
However, in the case where the detection information from a sensor is intermittently received and a signal is transmitted without synchronizing the sensor with the alarm unit in order to reduce an electric power consumption of the alarm unit, the time interval from the transmission of the sensor signal until an alarm is generated becomes unstable and also varies in dependence upon the receiving condition.
In the case where the abnormal condition such as leakage of gas, fire or the like occurs when a family is away from home, even if an alarm is generated in a room, it will be meaningless. Therefore, an alarm system whereby an alarm unit is attached at a location out of the room, e.g., a corridor or the like and thereby to inform the occurrence of abnormal situation to the outside has been put in a practical use.
However, in case of installing alarm units both indoors and outdoors, it is necessary to connect the alarm unit in the room with the alarm unit out of the room through a signal line. In addition, a power source has to be supplied from inside the room to the outside alarm unit. Consequently, in the alarm system whereby the alarm units are installed both inside and outside of the room, a construction to install a signal line by forming a hole in the wall has to be performed in order to connect both units with the signal line, so that this results in complication of the construction for installation and causes an economical demerit.
On the other hand, a problem on a power supply for the alarm unit which is attached outdoors can be solved by use of a battery in the outdoor unit. However, the service life of the battery is generally about one year and, for example, in the case where the battery energy is completely consumed when a family is away from home, the function of the outdoor alarm unit will have been lost; thus, periodical maintenance is needed.